Publicerad i IMPACT nummer 37, s 18-19, ISSN 2045-014

If only the world were static Most of our roads have been planned and designed using static assignment models. These are models that were designed by mathematicians and as such they had to have properties that mathematicians like: they had to be differentiable, continuous and in general present a convex, solvable problem of how people get from A to B and what that means for traffic on the roads. Unfortunately this also meant they had to assume that roads behave something like balloons, being able to expand to accommodate any amount of traffic, be it at an asymptotically increasing amount of travel time for those vehicles. Reality works rather different. Roads have an absolute maximum capacity and travel times increase suddenly when this capacity is approached. In addition, adding only a few more vehicles than a road can take will result in queues (called capacity breakdown) and the recovery capacity is much lower, meaning that the queues will only disappear when the traffic decreases far below the original maximum capacity. While it is clear why planners prefer models that do not suddenly produce chaos while they are planning the road network, unfortunately real traffic does behave in this way. In addition, these models usually work on a whole day’s worth of traffic and assume that traffic demand and road capacity are static for the whole day. In effect this presumes that traffic can be evenly spread over the whole day, nobody has to be anywhere at a specific time, and rush hours do not exist. While these assumptions greatly simplified the problem and made it possible to make computer models that would run on the computers available in the eighties and nineties, it also means that many of the current roads that were planned with such models never come close to being able to deal with the current traffic, especially when it counts: in the morning and afternoon rush hours.